1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to cable modem communication systems.
2. Description of Related Art
Data communication systems have been under continual development for many years. Cable modem (CM) communication systems have been of particular interest in the past several years, given their operable bandwidth and data rates being significantly greater than many other forms of communication systems. This is particular in the case of Internet access for individual subscribers. A CM communication system typically involves communication between a CM and a cable modem termination system (CMTS). The upstream within the CM communication system often involves the changing of upstream communication (from CM to CMTS) from one channel to another. In a typical CM communication system employing the Data Over Cable Service Interface Specification (DOCSIS), the original channel has undergone already initialization and ranging processes to provide for optimal upstream communication performance on that particular channel.
Cable-based communications systems are used to carry many types of information, including video programming, voice services, data services, etc. Data services may themselves include video, audio, voice, and other real-time services as well as best-effort Internet Protocol (IP) services such as email, web surfing, and file transfer. DOCSIS is a commonly used standard for data communication in cable systems. DOCSIS is intended to guarantee interoperability among equipment from different vendors. It specifies the behavior of the cable data communication system and its parts on a number of levels. It includes system and plant architecture requirements; physical-layer specifications covering the transmission of physical signals on the cable plant, including frequency plan, modulation, coding, fidelity requirements, etc.; Medium Access Control (MAC) layer specifications covering the format, timing, and management of data transmissions, including packet formats, management messaging, error handling, et al.; and specifications for interfaces in areas such as connection of a users' computer, connection of headend equipment to other networks (e.g. a WAN or the Internet), network management functionality, et al.
In a DOCSIS system, a single Cable Modem Termination System (CMTS) acts as a supervisory node. One or more Cable Modems (CMs) act as client nodes. The CMTS generally resides at a cable headend or other operator site, while the CMs reside at the customer premises. The CMTS transmits downstream data traffic in a broadcast manner, i.e. so that it is received by all CMs. Besides data associated with services being provided, this downstream data also includes various kinds of management messages that provide the CMs with MAC information such as when the CM is allowed to transmit, what physical layer parameters it must use, etc. The CM will use this information to transmit upstream data to the CMTS in a point-to-point fashion, i.e. only the CMTS can “hear” the transmissions of the CM. The CMTS manages the CMs in such a way as to guarantee that no CM's transmissions will interfere with those of another CM so that each CM's transmissions may be properly received (except in designated regions, known as “contention” regions, in which multiple CMs are allowed to transmit and may collide with each other). To guarantee this, DOCSIS provides for separation of CM transmissions in time, in frequency, or in codes.
As one tool for separating CM transmission, DOCSIS includes a construct called a channel. A channel is defined by an Upstream Channel Descriptor (UCD) message, a type of MAC-layer management message which is sent downstream by the CMTS to all CMs. A UCD includes a Channel ID (which is an arbitrary 8-bit identifier for the channel) and a number of parameters which define the physical-layer associated with a channel (e.g. center frequency, methods of coding, preamble length, etc.). In a given system, the CMTS may have any number of channels active; for each channel, it periodically sends a UCD message describing that channel. A particular CM will either choose a specific channel to operate on or be instructed by the CMTS to operate on a specific channel. Typically, a channel will have a large number (tens to hundreds) of CMs operating on it at the same time. All CMs on a single channel must use the same physical layer parameters, as specified by the UCD message.
In versions of DOCSIS prior to 2.0, the frequency plan of the various active channels is organized such that channels operating on the same physical segment of the cable plant used different center frequencies chosen such that there is little or no spectral overlap between channels, thus providing separation in frequency of groups of CMs. Within each channel, the CMTS then schedules the upstream transmissions of the various CMs in a Time Division Multiple Access (TDMA) fashion so that each CM received the desired number and frequency of transmit opportunities with no overlap between CMs (except for contention regions open to multiple CMs). The CMTS transmits a MAC layer management message known as an Upstream Bandwidth Allocation message, or MAP message, to indicate to the CMs the allocation of time slots on a particular channel.
A MAP message defines the use to which each time slot may be put on a particular channel. Separate MAP messages are sent for each channel. The MAP messages for a channel contain a Channel ID field matching that of the UCD messages for that same channel. A MAP message generally also includes information about time slots on the channel. This information includes: the slot's start time; its duration; the CM or CMs which are allowed to use that slot; and the type of transmission it or they may use the slot for. Transmission type is specified by an Interval Usage Code (IUC). Each IUC has a designated purpose, e.g. for requests, for long data transmissions, for short data transmissions, for maintenance activities, et al. When the MAP indicates that a particular CM may use a given time slot, the CM may transmit a burst of the specified type during that time slot. A burst is defined by the period during which the CMs transmitter is on. The CMs transmitter must be off during any timeslot in which the CMTS has not specifically given that CM (or a group of CMs to which it belongs) permission to transmit.
DOCSIS 2.0 adds new tools for separating the transmissions of the various CMs. One such tool is Synchronous Code Division Multiple Access (S-CDMA). With S-CDMA, transmissions from various CMs are still scheduled in time; however, at a given time, more than one CM may be physically transmitting using a particular set of codes. The codes chosen are orthogonal so that each CMs transmission may be independently recovered at the receiver, providing for separation and management of CM transmissions via codes.
DOCSIS 2.0 also introduces the concept of a “logical channel.” In contrast with DOCSIS 1.1, where each channel on a single physical plant segment must use a different center frequency, DOCSIS 2.0 allows the coexistence of multiple “logical channels” using the same spectrum on the same physical plant segment. Each logical channel is described by its own UCD message; this allows CMs on different logical channels to use different physical layer parameters (although all CMs on the same logical channel must use the same physical layer parameters). To prevent these logical channels from interfering with each other, the CMTS schedules the various logical channels using a particular spectrum for different time slots, so that at any given time only one such logical channel is transmitting, while the others are scheduled for idle slots during this time. Thus, the CMTS manages these logical channels in such a way as to separate them in time. The term “physical channel” is sometimes used to refer to the particular part of the available spectrum which is being shared among logical channels, while the term “logical channel” is used to refer to one of the channels as described by a UCD message which occupies the spectrum of the physical channel. The coexistence of multiple logical channels within a physical “channel” is completely transparent to the CM; thus, the concept of a “logical channel” is only meaningful at the CMTS, where the sharing of spectrum is visible. The CM behaves as instructed by the CMTS via the UCD and MAP messages for the channel, which contain no information about spectral sharing, and therefore the term “channel” (not “logical channel”) is applicable at the CM.
A UCD message defines a channel. In order to operate on a channel, a CM must receive a UCD message describing that channel. A UCD message contains two types of physical layer parameters: channel-wide parameters, which are used for all transmissions on the channel, regardless of burst type; and burst-specific parameters, which may be different for different types of bursts (i.e. for different IUCs). Examples of channel-wide parameters are center frequency, coding type (S-CDMA or TDMA), preamble pattern, et al. Examples of burst-specific parameters are modulation order (e.g. QPSK, 16QAM, 64QAM, and et al.), forward error correction (FEC) codeword size, number of FEC parity bytes, byte interleaver matrix size, et al. Burst-specific parameters are typically chosen so as to maximize the efficiency of each burst type; for example, a short data grant burst type may be specified to use a relatively short FEC codeword size to provide a reasonable level of error correction ability, but such codeword sizes would be very inefficient for a long data grant burst type and thus this type may use a relatively long FEC codeword size instead. The set of burst-specific parameters for all allowed burst types is known as the set of “burst profiles” for that channel. Burst profiles are properties of a channel; thus, all modems on a channel must use the same set of burst profiles. Thus, although long data bursts may use different parameters than short data bursts, a modem on a given channel must use the same parameters for transmitting short data bursts as all other modems on a given channel. Burst profiles may be chosen to balance any of a number of considerations, such as efficiency, robustness in the presence of certain types of noise, etc.
DOCSIS specifies that UCD messages containing the descriptions of the upstream channels in the system be sent periodically by the CMTS. In general, the periodic UCD messages describing a particular channel are always the same (they must be sent periodically to provide information about the channel to new CMs attempting to join the network). Thus, once established, the parameters of a particular channel (as described by a UCD message with a particular Channel ID) do not change. If the CMTS wishes to change the parameters of a channel (perform a “UCD change”), it must follow strict rules regarding notification of CMs of the upcoming change, timing of the change, and coordination of the change with MAP messages on the channel. A UCD change affects all CMs currently on the channel; i.e., all CMs on the channel must begin using the new parameters at the specified time.
When a CM first joins the network, it chooses a particular upstream channel on which to operate (or is instructed by the CMTS to operate on a particular upstream channel) and performs an initialization process. This initialization process includes a step known as ranging, whereby the CM and CMTS cooperate to determine what timing offsets the CM must apply to its transmission (based on the distance between CM and CMTS), what transmit pre-equalizer coefficients (if any) the CM must use when transmitting (based on the physical characteristics of the channel in use), and possibly other parameters individual to this CM. Once this ranging process is complete, the CM can transmit upstream data in a manner which will not interfere with other CMs and will be properly received at the CMTS. The initialization process may include other steps as well (e.g. authentication, registration on the network, etc.). When initialization is complete, the CMTS will allow the CM to pass data traffic on the channel. This data traffic may include best-effort services such as email or web traffic, and it may also include real time services such as voice (e.g., using VoIP [Voice over Internet Protocol]), video, audio, two-way video- or audio-conferencing, etc.
Sometimes it is desirable for the CMTS to instruct a particular CM to move from one channel to another. This may be done for a number of purposes. The operator may wish to perform “load balancing” by moving CMs from a heavily loaded channel onto a lightly loaded one. Or the operator may wish to perform system maintenance, perhaps involving the swapping or upgrading of headend equipment, which requires that a particular card, shelf, cabling segment, etc. be free of traffic. At the time the operator wishes to move a CM from one channel to another, the CM may or may not be actively passing traffic. In general, it is not possible to make channel changes only on CMs which are not actively passing traffic. This is particularly true in a system which provides real-time services, when a session (e.g. a phone call) may be in progress at the time the channel change is desired.
DOCSIS provides a mechanism called Dynamic Channel Change (DCC) for the purpose of moving a single CM from one channel to another. The messaging involved in this process is complex and affects many layers of the system. The process may be briefly summarized by the following steps: (1) CMTS instructs CM to change channels, and optionally provides the UCD parameters of the “new” channel and/or specifies which portions of the normal initialization process must be performed by the CM after it switches channels and before beginning to pass traffic on the new channel; (2) CM acknowledges receiving the channel change instruction; (3) CM stops transmitting on the old channel; (5) CM switches to the new channel and performs whatever initialization steps were specified by the CMTS; (6) CM begins transmitting on the new channel.
A problem with the current state of the art lies in the initialization steps. These steps must be performed in order for the CM to transmit successfully on the new channel without interfering with other CMs' transmissions, and may take several seconds or more to complete. During this initialization period, the CM is unable to transmit normal data traffic. If the CM is carrying real-time services, the gap in transmission due to initialization on the new channel may result in complete loss of the real-time connection (e.g. dropping of the phone call). This behavior is unacceptable in a modern communications system. Even if the gap is short enough that the connection is not dropped, it may result in lost packets and/or jitter on the periodicity of the packets, either or both of which could cause unacceptable degradation of the quality of the connection.
DOCSIS allows for the possibility of reducing the delay due to re-initialization on a new channel by allowing the CMTS to specify which initialization steps, if any, must be taken by the CM when it moves to the new channel. However, in many cases it may not be physically possible to omit certain initialization steps. For example, if a CM is instructed to move to a new channel with a substantially different center frequency, the transmit pre-equalizer coefficients needed by the CM to operate on this new channel may be very different from those which were used on the old channel, and thus a process of ranging is required to determine these coefficients before the CM can successfully transmit on the new channel. Similarly, the new channel may use burst profiles which are chosen for maximum efficiency (e.g. high order modulation, little FEC) but require great precision in transmit timing and/or equalization; this precision may not be physically achievable without a process of ranging and its inherent delay. Because of the physically necessity of re-initialization, there may be very few to zero combinations of channels between which a CM may perform a DCC while carrying real-time traffic without unacceptably degrading or dropping the real-time connection. This places a serious limitation on operators who wish to support such services.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.